noise from usb dac after powering down

[shadowlord]

i recently installed moode on a raspberry pi zero w to build a internet radio device for my stereo system.

installation and setup went quite well despite me being a novice.

for sound output i use a usb powered dac.

i have one small problem with the system though.
after powering down the unit via the shutdown command, the dac still makes some noise.
(i sound like a loud hum)
it seems the dac is still supplied with some power.

is there a way to get rid of this ?

thanks in advance and best regards
Chris